Display device including pixel comprising first transistor second transistor and light-emitting element

ABSTRACT

An active matrix display device having a pixel structure in which pixel electrodes, gate wirings and source wirings are suitably arranged in the pixel portions to realize a high numerical aperture without increasing the number of masks or the number of steps. The device comprises a gate electrode and a source wiring on an insulating surface, a first insulating layer on the gate electrode and on the source wiring, a semiconductor layer on the first insulating film, a second insulating layer on the semiconductor film, a gate wiring connected to the gate electrode on the second insulating layer, a connection electrode for connecting the source wiring and the semiconductor layer together, and a pixel electrode connected to the semiconductor layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/798,567, filed Jul. 14, 2015, now allowed, which is a continuation ofU.S. application Ser. No. 14/148,912, filed Jan. 7, 2014, now U.S. Pat.No. 9,099,361, which is a continuation of U.S. application Ser. No.13/325,129, filed Dec. 14, 2011, now U.S. Pat. No. 8,633,488, which is adivisional of U.S. application Ser. No. 12/837,117, filed Jul. 15, 2010,now U.S. Pat. No. 8,178,880, which is a continuation of U.S. applicationSer. No. 12/485,958, filed Jun. 17, 2009, now U.S. Pat. No. 7,781,770,which is a divisional of U.S. application Ser. No. 11/460,105, filedJul. 26, 2006, now U.S. Pat. No. 7,560,732, which is a continuation ofU.S. application Ser. No. 10/424,793, filed Apr. 29, 2003, now U.S. Pat.No. 7,084,019, which is a divisional of U.S. application Ser. No.09/840,641, filed Apr. 24, 2001, now U.S. Pat. No. 6,580,475, whichclaims the benefit of a foreign priority application filed in Japan asSerial No. 2000-128536 on Apr. 27, 2000, all of which are incorporatedby reference.

BACKGROUND OF THE INVENTION

Field of the Invention

This invention relates to a semiconductor device having an integratedcircuit constituted by thin-film transistors (hereinafter referred to asTFTs) and to a method of fabricating the same. The invention relates to,for example, an electro-optical device as represented a liquid crystaldisplay panel and an electronic device mounting the electro-opticaldevice as a part. In this specification, the semiconductor devicegenerally stands for such devices that function by utilizing thesemiconductor properties. Therefore, electro-optical devices,semiconductor circuits and electronic devices are all encompassed by thescope of the semiconductor device.

Related Art

Technology has been developed for fabricating thin-film transistors(TFTs) by using a thin semiconductor film (of a thickness of from aboutseveral nanometers to about several hundred nanometers). TFTs have beenput into practical use as switching elements of the liquid crystaldisplay devices and, in recent years, it is becoming possible to form aCMOS circuit and an integrated circuit on a substrate such as of aglass.

Active matrix liquid crystal display devices are becoming a main streamof liquid crystal display devices, by arranging pixel electrodes in theform of a matrix and by using TFTs as switching elements connected tothe pixel electrodes, in order to meet the demand for realizing a highlyfine picture quality. The active matrix liquid crystal display devicescan be roughly divided into two; i.e., those of the transmission typeand those of the reflection type. In particular, the liquid crystaldisplay device of the reflection type, which does not use back light,has a merit that it consumes electric power in smaller amounts than thetransmission-type liquid crystal display device, and is finding anincreasing demand as a direct view-type display device for portable dataterminals and video cameras.

The active matrix liquid crystal display device of the reflection typeselects a state where an incident beam is reflected by a pixel electrodeand is output to the outer side of the device and a state where theincident beam is not output to the outer side of the device by utilizingthe optical modulation action of the liquid crystals, produces a brightdisplay and a dark display, and, further, combines these displays todisplay a picture. In general, pixel electrodes in the liquid crystaldisplay device of the reflection type are formed of an electricallyconducting material having a high optical reflection factor such as ofaluminum or silver.

In any way, the size of each pixel becomes inevitably more fine as thepicture quality becomes more fine. As a result, the ratio of areasoccupied by the TFT, source wiring and gate wiring increases in thepixel portion, and the numerical aperture decreases. In order toincrease the numerical aperture of the pixels within a specified pixelsize, therefore, it is essential to efficiently lay out the circuitelements necessary for constituting the pixel circuit.

SUMMARY OF THE INVENTION

This invention was derived in view of the above-mentioned problem, andhas an object of providing an active matrix-type display device having apixel structure in which a pixel electrode, a gate wiring and a sourcewiring are suitably arranged in a pixel portion, and which realizes ahigh numerical aperture without increasing the number of the masks orthe number of the steps.

This invention has a structure in which TFTs are shut off the lightwithout using a light-shielding film that forms a black matrix, andlight leaking among the pixels is shut off. The above structure of thisinvention comprises;

-   -   a gate electrode and a source wiring over an insulating surface;    -   a first insulating film over the gate electrode and over the        source wiring;    -   a semiconductor film over the first insulating film;    -   a second insulating film over the semiconductor film;    -   a gate wiring connected to the gate electrode over the second        insulating film;    -   a connection electrode for connecting the source wiring and the        semiconductor film together; and    -   a pixel electrode connected to the semiconductor film.

Another invention has a constitution which comprises:

-   -   a first gate electrode, a second gate electrode and a source        wiring over an insulating surface;    -   a first insulating film over the first and second gate        electrodes and over the source wiring;    -   a first semiconductor film having a source region, a drain        region and a channel-forming region over the first insulating        film;    -   a second semiconductor film overlapped over the second gate        electrode;    -   a second insulating film over the first and second semiconductor        films;    -   a gate wiring connected to the gate electrode over the second        insulating film;    -   a connection electrode for connecting the source wiring and the        source region together; and    -   a pixel electrode connected to the drain region and to the        second semiconductor film.

In this constitution of the invention, the end on one side of the pixelelectrode is formed on the source wiring so as to also serve as alight-shielding film, enabling the pixel electrode to occupy anincreased area in the pixel unit.

A further invention has a constitution which comprises:

-   -   a first step of forming a gate electrode and a source wiring        over an insulating surface;    -   a second step of forming a first insulating film over the gate        electrode;    -   a third step of forming a semiconductor film over the first        insulating film;    -   a fourth step of forming a second insulating film over the        semiconductor film; and    -   a fifth step of forming, over the second insulating film, a gate        wiring connected to the gate electrode, a connection electrode        for connecting the source wiring and the semiconductor film        together, and a pixel electrode connected to the semiconductor        film.

A further invention has a constitution which comprises:

-   -   a first step of forming a gate electrode and a source wiring        over an insulating surface;    -   a second step of forming a first insulating film over the gate        electrode;    -   a third step of forming a semiconductor film over the first        insulating film;    -   a fourth step of forming a source region and a drain region over        the semiconductor film;    -   a fifth step of forming a second insulating film over the        semiconductor film; and    -   a sixth step of forming, over the second insulating film, a gate        wiring connected to the gate electrode, a connection electrode        for connecting the source wiring and the source region together,        and a pixel electrode connected to the drain region.

A further invention has a constitution which comprises:

-   -   a first step of forming a first gate electrode, a second gate        electrode and a source wiring over an insulating surface;    -   a second step of forming a first insulating film over the first        and second gate electrodes;    -   a third step of forming, over the first insulating film, a first        semiconductor film that overlaps over the first gate electrode        and a second semiconductor film that overlaps over the second        gate electrode;    -   a fourth step of forming a source region and a drain region in        the first semiconductor film;    -   a fifth step of forming a second insulating film over the        semiconductor film; and    -   a sixth step of forming, over the second insulating film, a gate        wiring connected to the gate electrode, a connection electrode        for connecting the source wiring and the source region together,        and a pixel electrode for connecting the drain region and the        second semiconductor film together.

According to the above steps, the end on one side of the pixel electrodeis formed over the source wiring to form a pixel structure in which thesource wiring also serves as a light-shielding film, enabling the pixelelectrode to occupy an increased area in the pixel portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view illustrating the constitution of a pixel in aliquid crystal display device of this invention;

FIG. 2 is a top view illustrating a step of fabricating a TFT in a drivecircuit and a pixel TFT;

FIG. 3 is a circuit diagram illustrating the constitution of the pixelin the liquid crystal display device;

FIGS. 4(A) to 4(D) are sectional views illustrating steps of fabricatingthe TFT in the drive circuit and the pixel TFT;

FIGS. 5(A) to 5(C) are sectional views illustrating steps of fabricatingthe TFT in the drive circuit and the pixel TFT;

FIGS. 6(A) to 6(C) are sectional views illustrating a step offabricating the TFT in the drive circuit and the pixel TFT;

FIG. 7 is a sectional view illustrating the constitution of a pixel in aliquid crystal display device of the reflection type;

FIG. 8 is a top view illustrating the constitution of the pixel in theliquid crystal display device of the reflection type;

FIG. 9 is a sectional view illustrating the constitution of a pixelportion in a liquid crystal display device of the transmission type;

FIG. 10 is a sectional view illustrating the constitution of a liquidcrystal display device;

FIG. 11 is a view illustrating how to assemble the principal constituentelements of the liquid crystal display device;

FIG. 12 is a view illustrating the constitution of a reflection typeliquid crystal display device using a front light;

FIGS. 13(A) to 13(C) are sectional views illustrating the constitutionof the pixel portion in an EL display device;

FIG. 14 is a top view illustrating the constitution of the pixel portionin the EL display device;

FIGS. 15(A) to 15(E) are views illustrating examples of thesemiconductor device; and

FIGS. 16(A) to 16(C) are views illustrating examples of thesemiconductor device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a pixel structure in the active matrix liquidcrystal display device is constituted by a gate wiring 235 arranged in adirection of the row, a source wiring 207 arranged in a direction of thecolumn, a pixel TFT provided at a portion where the gate wiring and thesource wiring intersect each other, and a pixel electrode connected tothe pixel TFT.

A gate electrode 204 of a TFT provided for every pixel (hereinafterreferred to as pixel TFT) is formed on an insulating surface, and asemiconductor film 212 is formed via a first insulating film. The sourcewiring 207 is formed on the same insulating surface as that of the gateelectrode 204. The gate wiring 235 and a pixel electrode 236 are formedon a second insulating film formed on the semiconductor film 212. Thegate wiring 235 and the pixel electrode 236 are connected to the gateelectrode 204 and to the semiconductor film 212, respectively, throughcontact holes. Further, the source wiring 207 and the semiconductor film212 are connected together through a connection wiring 234 formed on thesame layer as the gate wiring 235.

Due to this pixel structure, a portion where the semiconductor film 212intersects the gate electrode 204 (a portion where a channel is formedin the TFT) can be covered with the gate wiring 235 so as to be shieldedfrom the light. It is desired that other regions of the semiconductorfilm 212 are shielded from the light, too. In FIG. 1, the gate electrodeis of a comb shape to illustrate a TFT structure where there existplural channel-forming regions. It is, however, desired that even aregion existing between a channel-forming region and anotherchannel-forming region, too, is shielded from the light by the gatewiring 235.

In the pixel structure of FIG. 1, the gate electrode works as oneelectrode for forming a holding capacity in the pixel. A pixel is formedby the semiconductor film 212 and the pixel electrode 236 connectedthereto. Here, the pixel electrode 236 is further connected to thesemiconductor film 213. The semiconductor film 213 is formed as anelectrode for forming a capacity, and forms a holding capacity togetherwith the gate electrode 205. In this case, a p-type impurity (accepter)is added to the semiconductor film 213. In this constitution, theholding capacity is formed being overlapped on the pixel electrode 236.

Further, the end of the pixel electrode 236 on one side can be formedbeing overlapped on the source wiring 207 to shut off light that leaksthrough a gap to the neighboring pixel electrode 242.

A pattern of the above pixel structure can be formed by a process forexposure to light. The process for exposure to light requiresphotomasks, i.e., a first photomask for forming a gate electrode, asecond photomask for forming a semiconductor film, a third photomask forforming an LDD region of the pixel TFT, a fourth photomask for forming acontact hole, and a fifth photomask for forming the gate wiring, pixelelectrode and connection wiring. Namely, the pixel portion can be formedby using five pieces of photomasks. When a drive circuit is formedaround the pixel portion by applying a CMOS circuit constituted byn-channel TFTs and p-channel TFTs, an additional photomask is needed forcovering the n-channel TFTs. When the pixel structure shown in FIG. 1 isconstituted as described above, there can be realized a reflection-typeliquid crystal display device having a high pixel numerical apertureusing a small number of masks.

The pixel structure shown in FIG. 1 is fabricated based on aprerequisite of being used for the reflection-type liquid crystaldisplay device. However, if the transparent electrode is formed in apredetermined pattern by adding one more piece of photomask, it becomespossible to produce a liquid crystal display device of the transmissiontype, too. The thus constituted invention will be described in furtherdetail by way of embodiments described below.

EMBODIMENTS Embodiment 1

This embodiment deals with a method of simultaneously fabricating apixel portion and TFTs (n-channel TFT and p-channel TFT) for forming adrive circuit around the pixel portion on the same substrate withreference to the drawings.

Referring, first, to FIG. 4(A), on a substrate 201 made of a glass suchas barium borosilicate glass or alumino borosilicate glass asrepresented by a glass #7059 or #1737 of Corning Co., there are formedgate electrodes 202 to 204, source wirings 206, 207 and a capacitorwiring 205 for forming a holding capacity of a pixel portion of anelectrically conducting film containing one or plural kinds ofcomponents selected, preferably, from molybdenum (Mo), tungsten (W) andtantalum (Ta). An alloy of Mo and W is desired from the standpoint ofdecreasing the resistance and enhancing the heat resistance. The gateelectrodes may be formed by using aluminum while oxidizing the surfacethereof.

The gate electrodes formed by using a first photomask has a thickness of200 to 400 nm, preferably, 250 nm, and have ends that are tapered sothat a film can be favorably formed thereon (to improve step coverage).The ends are tapered at an angle of 5 to 30 degrees and, preferably, 15to 25 degrees. The ends are tapered by dry-etching and the angles arecontrolled relying on an etching gas and a bias voltage applied to thesubstrate side.

Referring next to FIG. 4(B), a first insulating film 208 is formed forcovering the gate electrodes 202 to 204, source wirings 206, 207 andcapacitor wiring 205 that forms a holding capacitor in the pixelportion. The first insulating film 208 is the one containing silicon andis formed maintaining a thickness of 40 to 200 nm by the plasma CVDmethod or the sputtering method. The first insulating film 208 is formedof, for example, a silicon nitride film 208 a of a thickness of 50 nmand a silicon oxide film 208 b of a thickness of 120 nm. It is furtherallowable to use a silicon oxinitride film (SiO_(x)N_(y)) formed ofSiH₄, N₂O and NH₃ by the plasma VD method.

The first insulating film 208 includes a semiconductor film formed as anupper layer thereof and is used as a gate-insulating film, and furtherexhibits a function of a blocking layer to prevent the diffusion ofimpurities such as of an alkali metal in the semiconductor film from thesubstrate 201.

The semiconductor film 209 is formed on the first insulating film 208 byusing a polycrystalline semiconductor maintaining a thickness of 30 to100 nm and, preferably, 40 to 60 nm. Though there is no limitation onthe material of the polycrystalline semiconductor, there can betypically used silicon or a silicon-germanium (SiGe) alloy. Thepolycrystalline semiconductor is obtained by subjecting a semiconductorhaving an amorphous structure formed by the plasma CVD method orsputtering method to the crystallization relying upon a lasercrystallization method or thermal crystallization method.

The polycrystalline semiconductor is formed by the laser crystallizationmethod by using an excimer laser, a YAG laser, a YVO₄ laser or a YLFlaser of the pulse oscillation type or of the continuous emission type.When these lasers are used, the laser beam emitted from the laseroscillator is linearly collected through an optical system and isprojected onto the semiconductor film. The crystallization conditionscan be suitably selected by a person who conducts the production. Whenthe excimer laser is used, however, the pulse oscillation frequency isset to be 30 Hz and the laser energy density is selected to be 100 to400 mJ/cm² (typically, 200 to 300 mJ/cm²). When the YAG laser is used,the pulse oscillation frequency is set to be 1 to 10 kHz by using thesecond harmonics and the laser energy density is set to be 300 to 600mJ/cm² (typically, 350 to 500 mJ/cm²). A laser beam linearly focusedinto a width of 100 to 1000 μm and, for example, into 400 μm isprojected onto the whole surface of the substrate at an overlappingratio of the linear laser beam of 80 to 98%.

At this step, a p-type impurity (acceptor) as represented by boron maybe added to the semiconductor film 209 at a concentration of 1×10¹⁶ to5×10¹⁷/cm³ in order to control the threshold voltage of the TFTs.

The semiconductor film 209 of the polycrystalline semiconductor isformed in a predetermined pattern by using a second photomask. FIG. 4(C)illustrates semiconductor films 210 to 213 divided into islands.Semiconductor films 210 to 212 are so formed as will be partlyoverlapped on the gate electrodes 202 and 204. FIG. 2 is a top view of apixel portion in this state, and FIG. 4(C) is a sectional view along theline A-A′ of FIG. 2.

Thereafter, an insulating film of silicon oxide or silicon nitride isformed maintaining a thickness of 100 to 200 nm on the semiconductorfilms 210 to 213. Referring to FIG. 4(D), third insulating layers 214 to218 that serve as channel protection films are formed on thesemiconductor films 210 to 212 in a self-aligned manner by an exposureprocess from the back surface using the gate electrodes as a mask.

Then, a first doping step is effected to form an LDD (lightly dopeddrain) region of the n-channel TFT. The doping may be effected by theion doping method or the ion injection method. Phosphorus (P) is addedas the n-type impurity (donor), and first impurity regions 219 to 222are formed by using the third insulating layers 215 to 218 as a mask.The donor concentration in these regions is 1×10¹⁶ to 2×10¹⁷/cm³.

A second doping step is the one for forming a source region and a drainregion of the n-channel TFT. Referring to FIG. 5(A), masks 223 to 225are formed by the resist by using a third mask. The masks 224 and 225are formed covering the LDD region of the n-channel TFT, and a donorimpurity is added to the second impurity regions 226 to 228 at aconcentration in a range of 1×10²⁰ to 1×10²¹/cm³.

Before or after the second doping step, it is desired that the etchingis effected with a hydrofluoric acid in a state where the masks 223 to225 are formed to remove the third insulating layers 214 and 218.

Referring to FIG. 5(B), the source region and the drain region of thep-channel TFT are formed by a third doping step; i.e., a p-type impurity(acceptor) is added by the ion doping method or the ion injection methodto form third impurity regions 230 and 231. The p-type impurityconcentration in these regions is 2×10²⁰ to 2×10²¹/cm³. In this step,the p-type impurity is added to the semiconductor film 213, too.

Referring, next, to FIG. 5(C), a second insulating film is formed on thesemiconductor film. Preferably, the second insulating film is formed ofplural insulating films. A first layer 232 of the second insulating filmformed on the semiconductor film is an inorganic insulator of ahydrogen-containing silicon nitride film or a silicon oxinitride filmand has a thickness of 50 to 200 nm. Thereafter, the impurities added tothe semiconductor films are activated. This step is effected by aheat-annealing method using an annealing furnace. There can be furtheremployed a laser annealing method or a rapid thermal annealing method(RTA method). The heat-annealing method is conducted in a nitrogenatmosphere at 400 to 600° C. and, typically, at 450 to 500° C. for 1 to4 hours.

Due to this heat treatment, hydrogen is released from the siliconnitride film or the silicon oxinitride film which is the first layer 232of the second insulating film simultaneously with the activation of theimpurity element, and the semiconductor film is hydrogenated. This is astep to terminate the dangling bond of the semiconductor film withhydrogen. As means for efficiently executing the hydrogenation, theremay be executed a plasma hydrogenation (using hydrogen excited byplasma) prior to forming the first layer 232 of the second insulatingfilm.

A second layer 233 of the second insulating film shown in FIG. 6(A) isformed of an organic insulating material such as polyimide or acrylicmaterial, and has a flat surface. It is, of course, allowable to form asilicon oxide film of TEOS (tetraethyl orthosilicate) by the plasma CVDmethod. From the standpoint of enhancing the flatness, however, it isdesired to use the above organic material.

Then, contact holes are formed by using a fifth photomask. There arefurther formed a connection electrode 234 and source or drain wirings235, 236 in the drive circuit 305 by using aluminum (Al), titanium (Ti)or tantalum (Ta) using a sixth photomask. There are further formed apixel electrode 240, a gate wiring 239 and a connection electrode 238 ina pixel portion 306.

Thus, there are formed on the same substrate the drive circuit 305having a p-channel TFT 301 and an n-channel TFT 302, and the pixelportion 306 having a pixel TFT 303 and a holding capacitor 304. In thep-channel TFT 301 in the drive circuit 305, there are formed achannel-forming region 307 and a source or drain region 308 which is athird impurity region. In the n-channel TFT 302, there are formed achannel-forming region 309, an LDD region 310 which is a first impurityregion, and a source or drain region 311 which is a second impurityregion. The pixel TFT 303 in the pixel portion 306 is of a multi-gatestructure, and in which are formed a channel-forming region 312, an LDDregion 313, and source or drain regions 314 and 316. The second impurityregion located between the LDD regions 313 is effective in lowering theoff current. A holding capacitor 304 is formed by the capacitor wiring205, the semiconductor film 213 and the first insulating film formedtherebetween.

In the pixel portion 306, the source wiring 207 is electricallyconnected through a connection electrode 238 to the source or drainregion 314 of the pixel TFT 303. Further, the gate wiring 239 iselectrically connected to the first electrode. The pixel electrode 240is connected to the source or drain region 316 of the pixel TFT 303 andto the semiconductor film 213 of the holding capacitor 304.

FIG. 6(A) is a sectional view of the pixel portion 306 along the lineA-A′ of FIG. 1. Further, FIGS. 6(B) and 6(C) are sectional views alongthe lines B-B′ and C-C′ of FIG. 1. FIG. 6(B) is a view illustrating aportion where the gate electrode 204 and the gate wiring 239 arecontacted to each other. The gate electrode 204 serves as one electrodeof the holding capacitor in the neighboring pixel, and is forming acapacitor at a portion overlapped on the semiconductor film 244 that isconnected to the pixel electrode 245. FIG. 6(C) illustrates arelationship of arrangement among the source wiring 207, pixel electrode240 and neighboring pixel electrode 246. An end of the pixel electrodeis formed on the source wiring 207 to form an overlapped portion therebyto enhance the light-shielding performance by shutting off stray light.FIG. 3 shows an equivalent circuit of the above pixel. In thisspecification, the above substrate is referred to as active matrixsubstrate for convenience.

One of the advantages of forming the TFTs in an inverse staggering typeis that the LDD region overlapped on the gate electrode in the n-channelTFT can be formed in a self-aligned manner by the back-surface exposureprocess, and the dispersion in the TFT characteristics can be minimizedin addition to the feature of continuously forming the gate insulatingfilm and the semiconductor film.

Embodiment 2

This embodiment deals with a pixel structure applied to a liquid crystaldisplay device of the reflection type, which will now be described withreference to FIG. 8 which is a top view of the pixel and FIG. 7 which isa sectional view along the line D-D′ in FIG. 8. In FIG. 7, a pixel TFT420 in a pixel portion 422 includes a gate electrode 402, a firstinsulating film 405, a semiconductor film 406, a channel protection film408, second insulating films 409, 410, a source wiring 404, a gatewiring 412, a connection wiring 411 and a pixel electrode 413 formed ona substrate 401. A holding capacitor 421 is constituted by a capacitorwiring 403, a semiconductor film 407 and the first insulating film 405formed therebetween. The constitution thereof is the same as the pixelTFT 303 and the holding capacitor 304 shown in FIG. 6(A).

Island-like regions 417 to 419 are formed under the pixel electrode 413of the pixel portion 422 to render the surface thereof rugged. In FIG.7, three island-like regions are shown having sizes of 5 to 10 μm andmaintaining a pitch of 10 to 20 μm. The island-like regions areconstituted by first layers 417 a to 419 a formed by using the same filmas the gate electrode 402, second layers 417 b to 419 b formed by thesame layer as the semiconductor film 406, and third layers 417 c to 419c formed by the same layer as the third insulating layer 408. Theselayers are formed by etching through separate steps and are, hence,formed to become narrower toward the upper layers with their ends notbeing in agreement with one another.

Second insulating films 409 and 410 are formed thereon. Here, the secondinsulating film 410 is formed of an organic resin material to reflectthe ruggedness of the underlying layer. For this purpose, the secondinsulating film 410 is formed by applying an organic resin materialhaving a viscosity of 10 to 1000 cp (preferably, 40 to 200 cp) so as toform ruggedness on the surface. Upon forming the layer of the organicresin material, the surface becomes rugged with a mild curvature of aradius of curvature of 0.1 to 4 μm. Though FIG. 8 shows island regionsof a circular shape, the island regions are in no way limited to theabove shape but may have any polygonal shape. Upon forming the pixelshaving the constitution as described above, mirror-surface reflection isprevented in the liquid crystal display device of the reflection type,and the quality of display can be improved, particularly, at the time ofwhite display.

Embodiment 3

The embodiment 1 has dealt with the active matrix liquid crystal displaydevice of the reflection type. By forming the pixel electrode using atransparent electrically conducting film, however, it is possible toform a display device of the transmission type. A pixel TFT 383 in apixel portion 386 shown in FIG. 9 is fabricated in the same manner asthe pixel TFT 303 described in the embodiment 1, and this embodimentdescribes concerning the differences only.

After a second layer 229 of the second insulating film is formed byusing the organic resin material, first pixel electrodes 250 and 251 areformed simultaneously with the gate wiring and the connection electrode.The first pixel electrode 250 is connected to the semiconductor film ofthe pixel TFT 383, and the first pixel electrode 251 is connected to thesemiconductor film forming the holding capacitor 384. Thereafter, atransparent electrically conducting film 252 is formed to form a pixelelectrode.

The transparent electrically conducting film is formed by sputtering orvacuum-vaporizing indium oxide (In₂O₃) or an indium oxide-tin oxide(In₂O₃—SnO₂; ITO) alloy. The above material is etched by using ahydrochloric acid solution. Further, etching the ITO tends to produceresidue. In order to improve workability by etching, therefore, theremay be used an indium oxide-zinc oxide alloy (In₂O₃—ZnO). The indiumoxide-zinc oxide alloy exhibits excellent surface smoothness andsuperior thermal stability to ITO. Similarly, zinc oxide (ZnO) is apreferred material, too. In order to improve transmission factor forvisible light and electric conductivity, further, there can be used zincoxide (ZnO:Ga) to which gallium (Ga) is added.

In the embodiment 1, the active matrix substrate was prepared by using 5pieces of photomasks to fabricate a liquid crystal display device of thereflection type. However, by adding another piece of photomask (a totalof 6 pieces of photomasks), as described above, there can be prepared anactive matrix substrate that meets a liquid crystal display device ofthe transmission type.

Embodiment 4

This embodiment deals with the steps of fabricating an active matrixliquid crystal display device by using the active matrix substrateobtained in Embodiment 1. FIG. 10 illustrates a state where an activematrix substrate and an opposing substrate 454 are stuck to each otherwith a sealing member 458. First, pole-like spacers 451, 452 are formedon the active matrix substrate in the state of FIG. 6(A). The spacer 451provided on the pixel portion is overlapped on a contact portion on thepixel electrode. The spacer has a height of 3 to 10 μm though it mayvary depending upon the liquid crystal material that is used. In thecontact portion, a recessed portion is formed to correspond to thecontact hole. Upon forming the spacer to meet this portion, disturbancein the orientation of liquid crystals can be prevented. Thereafter, anorientation film 453 is formed followed by rubbing. A transparentelectrically conducting film 455 and an orientation film 456 are formedon the opposing substrate 454. Thereafter, the active matrix substrateand the opposing substrate are stuck together, and liquid crystals arepoured therein.

FIG. 11 schematically illustrates the assembling by sticking the activematrix substrate and the opposing substrate together. On the activematrix substrate 650 have been formed a pixel portion 653, a drivecircuit 652 on the scanning line side, a drive circuit 651 on the signalline side, an external input terminal 654, and a wiring 659 forconnecting the external input terminals to the input units of thecircuits. On the opposing substrate 655 are formed opposing electrodes656 to correspond to the regions where the pixel portions and the drivecircuits have been formed on the active matrix substrate 650. The activematrix substrate 650 and the opposing substrate 655 are stuck togethervia the sealing member 657, and liquid crystals are poured to form aliquid crystal layer 658 on the inside of the sealing member 657.Further, an FPC (flexible printed circuit board) 660 is stuck to theexternal input terminal 654 of the active matrix substrate 650. Areinforcing plate 659 may be provided to enhance the adhering strengthof the FPC 660.

The thus fabricated liquid crystal display device of the active matrixtype can be used as a display device for various electronic devices.Further, the method of fabricating the active matrix liquid crystaldisplay device of this embodiment can similarly be applied even infabricating the active matrix substrate of the embodiment 2 or of theembodiment 3.

Embodiment 5

FIG. 12 illustrates an example of when the active matrix liquid crystaldisplay device of the reflection type fabricated by using the pixelstructure of this invention is used as a direct view-type displaydevice. On the active matrix substrate 1203 are formed a pixel portion1201 and a drive circuit portion 1202, an opposing substrate 1204 isadhered thereto with a sealing member 1206, and a liquid crystal layer1205 is formed therebetween.

FIG. 12 illustrates the constitution of a liquid crystal display deviceof the reflection type using a front light and in which a front lightsystem 1208 is provided on a polarizer plate 1207. In a bright placesuch as in the day time, the liquid crystal display device of thereflection type displays the picture by utilizing external light. Whenthe external light cannot be introduced to a sufficient degree such asat night, the front light is used to produce the display. In any way, byemploying the pixel structure of this invention, the pixel electrodeoccupies an increased ratio of the pixel portion, and a bright displayof picture is realized. When the front light is used, light of a smallintensity suffices for the illumination making it possible to decreasethe amount of electric power consumed by an electronic device in whichthe liquid crystal display device is incorporated. The constitution ofthis embodiment can be applied to the active matrix liquid crystaldisplay device fabricated in the embodiment 4.

Embodiment 6

This embodiment deals with the case where the active matrix substrate ofthe embodiment 1 is applied to a self-light-emitting display device byusing an electro luminescence (EL) material (hereinafter referred to asEL display device). The electro luminescence material emits light byeither fluorescence or phosphorescence. The emission of light referredto in this embodiment includes either one of them or both of them.

FIG. 13 is a sectional view of the pixel portion in which there areformed a switching TFT 701, a current-controlling TFT 702 and a holdingcapacitor 703. These TFTs are formed through the same steps as those ofthe embodiment 1. The switching TFT 701 is an n-channel TFT, and has achannel-forming region 704, an LDD region 705 and a source or drainregion 706 formed in a semiconductor film 755 on a gate electrode 751.The semiconductor film 755 is connected to a source wiring 753 through aconnection electrode 761.

The current-controlling TFT 702 is a p-channel TFT, and has achannel-forming region 707 and a source or drain region 708 in asemiconductor film 756 on a gate electrode 752. The source side of thecurrent-controlling TFT 702 is connected to a power source line 764, andthe drain side thereof is connected to a drain electrode 765. To thedrain electrode 765 is connected a pixel electrode 766 which is formedof a transparent electrically conducting film. Further, a holdingcapacitor 703 is formed in a region where the capacitor wiring 752 andthe semiconductor film 756 are overlapped one upon the other.

The first insulating films 754 (754 a, 754 b) and second insulatingfilms 759, 760 are the same as those of the embodiment 1.

FIG. 13(A) is a sectional view along the line E-E′ in FIG. 14. FIGS.13(B) and 13(C) are sectional views along the line F-F′ and G-G′ in FIG.14, FIG. 13(B) illustrating a portion where a gate electrode 751 of theswitching TFT 701 is contacting to the gate wiring 772, and FIG. 13(C)illustrating a relationship of arrangement among the source wiring 753,the pixel electrode 767 and the neighboring pixel electrode 771, an endof the pixel electrode being formed on the source wiring 753 to form anoverlapped portion thereby to enhance the light-shielding performance.

In the pixel portion, there is formed a bank 767 which is an insulatingfilm covering an end of the pixel electrode which is an anode, and anorganic compound layer is formed thereon to produce electroluminescence.By applying the solution, there are formed a light-emitting layer ofsuch a material as polyvinyl carbazole and organic compound layers 768,769 inclusive of an electron-pouring layer of potassium acetyl acetonate(hereinafter referred to as acac K). A cathode 770 formed of an aluminumalloy is formed thereon. In this case, the cathode 770 also works as apassivation film. Thus, there is formed a self-light-emitting EL elementcomprising an anode, an organic compound layer and a cathode. In thecase of this embodiment, light emitted from the light-emitting layer 768travels toward the active matrix substrate.

Upon employing the pixel structure of this invention as described above,it is allowed to improve the numerical aperture of theself-light-emitting display device of the active matrix type, too. As aresult, the picture is displayed brightly and vividly.

Embodiment 7

This embodiment deals with a semiconductor device incorporating thedisplay device of this invention. Examples of the semiconductor deviceof this type include portable data terminals (electronic notebook,mobile computer, cell phone, etc.), video camera, still camera, personalcomputer, TV and the like as shown in FIGS. 15 and 16.

FIG. 15(A) illustrates a cell phone constituted by a main body 2901, avoice output unit 2902, a voice input unit 2903, a display device 2904,an operation switch 2905 and an antenna 2906. This invention can beapplied to the display device 2904. In particular, the liquid crystaldisplay device of the reflection type of the embodiment 5 is suited fromthe standpoint of decreasing the consumption of electric power.

FIG. 15(B) illustrates a video camera constituted by a main body 9101, adisplay device 9102, a voice input unit 9103, an operation switch 9104,a battery 9105 and an imaging portion 9106. This invention can beapplied to the display device 9102. In particular, the liquid crystaldisplay device of the reflection type of the embodiment 5 is suited fromthe standpoint of decreasing the consumption of electric power.

FIG. 15(C) illustrates a mobile computer or a portable data terminalconstituted by a main body 9201, a camera portion 9202, an imagingportion 9203, an operation switch 9204 and a display device 9205. Thisinvention can be applied to the display device 9205. In particular, theliquid crystal display device of the reflection type of the embodiment 5is suited from the standpoint of decreasing the consumption of electricpower.

FIG. 15(D) illustrates a receiver unit constituted by a main body 9401,a speaker 9402, a display device 9403, a receiver unit 9404 and anamplifier unit 9405. This invention can be applied to the display device9403. In particular, the liquid crystal display device of the reflectiontype of the embodiment 5 is suited from the standpoint of decreasing theconsumption of electric power.

FIG. 15(E) illustrates an electronic book constituted by a main body9501, display devices 9502, 9503, a storage medium 9504, an operationswitch 9505, and an antenna 9506. The electronic book displays the datastored in a mini-disk (MD) or in a DVD and the data received by theantenna. As the direct view type display deices 9502 and 9503, inparticular, the liquid crystal display device of the reflection type ofthe embodiment 5 is suited from the standpoint of decreasing theconsumption of electric power.

FIG. 16(A) illustrates a personal computer which is constituted by amain body 9601, a picture input unit 9602, a display device 9603 and akeyboard 9604. This invention can be applied to the display device 9603.In particular, the liquid crystal display device of the reflection typeof the embodiment 5 is suited from the standpoint of decreasing theconsumption of electric power.

FIG. 16(B) illustrates a player which uses a recording medium recordinga program (hereinafter called recording medium) and is constituted by amain body 9701, a display device 9702, a speaker unit 9703, a recordingmedium 9704 and an operation switch 9705. This device uses a DVD(digital versatile disk) and a CD as recording media, and can be usedfor appreciating music, appreciating movies, enjoying games andInternet. This invention can be applied to the display device 9702. Inparticular, the liquid crystal display device of the reflection type ofthe embodiment 5 is suited from the standpoint of decreasing theconsumption of electric power.

FIG. 16(C) illustrates a digital camera which is constituted by a mainbody 9801, a display device 9802, an eyepiece 9803, an operation switch9804 and an imaging portion (not shown). This invention can be appliedto the display device 9802. In particular, the liquid crystal displaydevice of the reflection type of the embodiment 5 is suited from thestandpoint of decreasing the consumption of electric power.

The pixel structure of the present invention enables the pixel electrodeto occupy an increased proportion of the pixel portion and, hence, makesit possible to improve the numerical aperture in the active matrixliquid crystal display device of the reflection type. As a result, thepicture can be brightly and vividly displayed at any portion of theliquid crystal display device of the reflection type.

What is claimed is:
 1. A display device including a pixel, the pixelcomprising: a first transistor; a second transistor; a light-emittingelement, a first wiring; a second wiring; and a third wiring, whereinone of a source and a drain of the first transistor is electricallyconnected to the first wiring, wherein a gate of the first transistor iselectrically connected to the second wiring, wherein one of a source anda drain of the second transistor is electrically connected to a pixelelectrode, wherein the other one of the source and the drain of thesecond transistor is electrically connected to the third wiring, whereinthe first wiring comprises a first portion and a second portion, whereinthe second wiring comprises a third portion and a fourth portion,wherein the third wiring comprises a fifth portion, wherein the firstportion and the third portion are positioned on a first insulatingsurface, and wherein the second portion, the fourth portion, and thefifth portion are positioned on a second insulating surface.
 2. Thedisplay device including the pixel according to claim 1, wherein theother one of the source and the drain of the first transistor iselectrically connected to the gate of the second transistor.
 3. Thedisplay device including the pixel according to claim 1, wherein thethird wiring is configured to supply power potential.
 4. The displaydevice including the pixel according to claim 1, wherein the gate of thefirst transistor comprises a first gate and a second gate.
 5. Thedisplay device including the pixel according to claim 1, wherein thefirst portion is parallel to the third portion.
 6. The display deviceincluding the pixel according to claim 1, wherein the second insulatingsurface is positioned over the first insulating surface.
 7. A displaydevice including a pixel, the pixel comprising: a first transistor; asecond transistor; a light-emitting element, a first wiring; a secondwiring; and a third wiring, wherein one of a source and a drain of thefirst transistor is electrically connected to the first wiring, whereina gate of the first transistor is electrically connected to the secondwiring, wherein one of a source and a drain of the second transistor iselectrically connected to a pixel electrode, wherein the other one ofthe source and the drain of the second transistor is electricallyconnected to the third wiring, wherein the first wiring comprises afirst portion and a second portion, wherein the second wiring comprisesa third portion and a fourth portion, wherein the third wiring comprisesa fifth portion, wherein the second portion, the fourth portion, and thefifth portion are positioned on a second insulating surface.
 8. Thedisplay device including the pixel according to claim 7, wherein theother one of the source and the drain of the first transistor iselectrically connected to the gate of the second transistor.
 9. Thedisplay device including the pixel according to claim 7, wherein thethird wiring is configured to supply power potential.
 10. The displaydevice including the pixel according to claim 7, wherein the gate of thefirst transistor comprises a first gate and a second gate.
 11. Thedisplay device including the pixel according to claim 7, wherein thefirst portion is parallel to the third portion.
 12. The display deviceincluding the pixel according to claim 7, wherein the second insulatingsurface is positioned over a first insulating surface.
 13. A displaydevice including a pixel, the pixel comprising: a first transistor; asecond transistor; a light-emitting element, a first wiring; a secondwiring; and a third wiring, wherein one of a source and a drain of thefirst transistor is electrically connected to the first wiring, whereina gate of the first transistor is electrically connected to the secondwiring, wherein one of a source and a drain of the second transistor iselectrically connected to a pixel electrode, wherein the other one ofthe source and the drain of the second transistor is electricallyconnected to the third wiring, wherein the first wiring comprises afirst portion and a second portion, wherein the second wiring comprisesa third portion and a fourth portion, wherein the third wiring comprisesa fifth portion, and wherein the fourth portion and the fifth portionare positioned on a second insulating surface.
 14. The display deviceincluding the pixel according to claim 13, wherein the other one of thesource and the drain of the first transistor is electrically connectedto the gate of the second transistor.
 15. The display device includingthe pixel according to claim 13, wherein the third wiring is configuredto supply power potential.
 16. The display device including the pixelaccording to claim 13, wherein the gate of the first transistorcomprises a first gate and a second gate.
 17. The display deviceincluding the pixel according to claim 13, wherein the first portion isparallel to the third portion.
 18. The display device including thepixel according to claim 13, wherein the second insulating surface ispositioned over a first insulating surface.
 19. A display deviceincluding a pixel, the pixel comprising: a first transistor; a secondtransistor; a capacitor; and a light-emitting element, a first wiring; asecond wiring; and a third wiring, wherein one of a source and a drainof the first transistor is electrically connected to the first wiring,wherein a gate of the first transistor is electrically connected to thesecond wiring, wherein one of a source and a drain of the secondtransistor is electrically connected to a pixel electrode, wherein theother one of the source and the drain of the second transistor iselectrically connected to the third wiring, wherein the gate of thesecond transistor is electrically connected to a first electrode of thecapacitor, wherein the first wiring comprises a first portion and asecond portion, wherein the second wiring comprises a third portion anda fourth portion, wherein the third wiring comprises a fifth portion,wherein the first portion, the third portion, the gate of the firsttransistor, the gate of the second transistor, and the first electrodeof the capacitor are positioned on a first insulating surface, andwherein the second portion, the fourth portion, and the fifth portionare positioned on a second insulating surface.
 20. The display deviceincluding the pixel according to claim 19, wherein the other one of thesource and the drain of the first transistor is electrically connectedto the gate of the second transistor, and wherein the other one of thesource and the drain of the first transistor is electrically connectedto the first electrode of the capacitor.
 21. The display deviceincluding the pixel according to claim 19, wherein the third wiring isconfigured to supply power potential.
 22. The display device includingthe pixel according to claim 19, wherein the gate of the firsttransistor comprises a first gate and a second gate.
 23. The displaydevice including the pixel according to claim 19, wherein the firstportion is parallel to the third portion.
 24. The display deviceincluding the pixel according to claim 19, wherein the first insulatingsurface is positioned over the second insulating surface.